1. Field of the Invention
The invention concerns a magneto-rheological (MR) damper, and more particularly, an MR damper that is readily adaptable to the conditions of the damping properties.
2. Description of the Related Art
MR dampers include a magneto-rheological fluid, a work volume to which the fluid is applied, and an element for generating a magnetic field, wherein the yield limit of the fluid may be modified by application of the magnetic field.
A housing cavity is divided by a piston into a piston-swept volume and a cylinder volume, with the fluid being capable of flowing between the volumes through openings. In principle, MR dampers operating in the Valve Mode, Shear Mode, or Squeeze Mode are known. Frequently the single principles are mixed, however, so that an accurate classification is not possible.
A MR damper operating, e.g., in the valve and shear modes is disclosed in patent specification U.S. Pat. No. 5,398,917 FIG. 3. The openings are formed between the moving piston and the housing inner wall. The fluid in the gap is subjected to the magnetic filed by an electrical coil radially arranged on the piston. In the event of axial movements of the piston, the fluid flows through the openings from the cylinder volume into the piston-swept volume or vice versa. Owing to the ratio of free cross-section/cylinder (piston-swept) volume <1, the movement of the piston is damped correspondingly. The opening acts as a valve, the effect of which may be increased or reduced by a change in viscosity.
Moreover the fluid flowing through the openings is sheared across the free cross-section on the stationary housing inner wall and the moving piston, resulting in additional damping.
It is a drawback in this solution that the coil is mounted on the piston, so that an adaptation of the MR damper to the conditions of the respective task (damping properties) by simple and rapid replacement of the coil is not possible. Optimum adaptation of the MR damper to different damping conditions accordingly is not possible.
It is another drawback of the known solution that damping takes place not so much through the viscosity of the fluid than rather the cross-section of the openings in the piston and the related throttling effect. In other words, the flow and creep properties of the magneto-rheological fluid are not put to optimum use.
It is another drawback of this solution that when a load is applied, the valve mode may possibly create a considerable pressure differential between the cylinder-swept volume and the cylinder volume, with all the components of the MR damper, in particular seals, having to be designed with corresponding adaptations.
An MR damper in which the coil is arranged externally of the housing cavity and which does not operate in the valve mode, is disclosed in U.S. Pat. No. 5,492,312. The MR damper operates in the squeeze mode, so that by application of a magnetic field by means of the coil, which extends through the full length of the MR damper, the entire housing cavity and thus the entire fluid is subjected to the magnetic field. Here the fluid may flow through an annular space between the piston and the housing inner wall between the cylinder volume and the piston-swept volume.
It is a drawback of this solution that the entire housing cavity is exposed to the magnetic field, with damping accordingly being achieved not so much by squeezing the fluid in the piston-swept volume or through the annular space, but rather by pinching of the piston in the fluid.